| Compared to the conventional activated sludge, aerobic granule sludge has the advantages of high biomass retention, excellent settleablility, wide diverse microbial species and efficient nutrients removal performance, therefore, aerobic granular sludge technology is regarded as one of efficient and innovative biotechnologies in wastewater treatment. However, the factors which could influence aerobic granulation are numerous, and the key factors are difficult to distinguish. In addition, matured granules often loose stability during long term operation. These serious problems limit the practical application of aerobic granular sludge technology. In this study, synthetic wastewater was used to simulate low strength domestic wastewater, grey relational analysis (GRA) and envorinmental parameters were employed to analyze the optimal cultivation conditions for aerobic granular sludge, and the long-term stability of aerobic granular sludge was enhanced through controlling the diameter of matured granules.In order to evaluate the correlation degree and influence order of impact factors on granulation of sludge, GRA was employed. The results showed that settling time (ST), aeration time (AT), superficial gas velocity (SGV), height/diameter (H/D) ratio and organic loading rates (OLR) were key impact factors that determine granulation of sludge. For sludge volume index (SVI) and granulation duration, the influence orders and the corresponding grey entropy relational grade were:SGV (0.9935)> AT (0.9921)> OLR (0.9894)>ST (0.9876)>H/D ratio (0.9857) and SGV (0.9928)> H/D ratio (0.9914)> AT (0.9909)> OLR (0.9897)> ST (0.9878), respectively. Compared to SVI transformation, relatively high SGV and H/D ratio fascinated the rapid cultivaiton of aerobic granular sludge.In order to further optimize the cycle duration of aerobic granules, the correlation between environmental parameters(pH, oxidation-reduction potential(ORP) and dissovled oxygen(DO)) and biochemical reactions was studied. The results showed that the plateau occured in pH and ORP profiles indicated the end of relevant biochemical ractions at anaerobic and aerobic stages, and anoxic stages should be terminated when DO exhausted, ORP reduced rapidly and pH reached a turning point. After the adjustment, the average diameter of aerobic granules reached 630?m on day 60. The complete granulation process consumed 75 days. Nitrogen compund and phosphate (PO43--P) in effluent were nearly quantitative after 50 days, and such good treatment performance maintained stable to the end of the experimental period.In addition, granulation of non-filamentous bulking sludge was realized after the adjustment of cycle duration and influent composition directed by pH, ORP and DO profiles. The accumulation of loosely bound polysaccharides was controlled, and the secretion of tightly bound polysaccharides and tightly bound protein was enhanced. Matured granules with average diameter of 600?m were obtained after 45 days.Matured aerobic granules often lose stability during long term operation. The increase of diameter was one of the reasons for breakage of aerobic granules. Based on optimal clutivation condition obtained above, combined selective pressure generated by low strength wastewater (chemical oxygen demand, COD,200±40 mg/L), low hydrodynamic shear force (superficial gas velocity of 0.55 cm/s and agitation of 180 rpm), different sludge discharge mode and the alternating anaerobic/aerobic/anoxic (A/O/A) operational process were employed to limit the diameter of aerobic granules. Granulation process was completed after 40 days. The matured granules with small diameter (d (0.9)< 800?m) were stable without disaggregation during the experimental period (220 days). The extracellular polysaccharide/ extracellular protein (PS/PN) ratio increased along with granulation process, and maintained at a stable level in the following time. Fluorescence In Situ Hybridization analysis showed that slow-growing ammonia oxidizing bacteria, nitrite oxidizing bacteria, phosphorus accumulating organisms and glycogen accumulating organisms were enriched in matured granules. After granulation, the removal efficiencies of COD, ammonium, total nitrogen (TN) and PO43--P reached 99%,98%,90% and 99%, respectively. The majority of total nitrogen was consumed during aerobic stage after complete granulationAn improved method was developed to evaluate the activity of functional organism with denitrification and/or PO43-P removal capability. The tests results showed that (1) aerobic phosphate uptake, aerobic denitrifying-phosphate uptake and aerobic denitrification occurred under aerobic condition, (2) anoxic denitrifying-phosphate uptake and anoxic denitrification occurred under anoxic condition. The aerobic phosphate uptake rate increased when nitrite (NO2--N,<4.8 mg N/L) or nitrate (NO3--N,-30 mg/L) was supplied, indicating that the traditional method which only supplied O2 to evaluate the capability of total PAOs was inaccurate. In addition, the sludge could perform denitrification even when phosphate was not supplied under aerobic condition (at anoxic layer of aerobic granules) or anoxic condition, suggesting that, besides denitrifying phosphorus accumulating organisms (DNPAOs), other denitrifying bacteria (such as denitrifying glycogen accumulating organisms and autotrophic denitrification bacteria) could also contribute to denitrification. Thus, the activity of other denitrifying organisms should be evaluated simultaneously to avoid overrating the denitrifying ability of DNPAOs. |
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